Descent from Heaven Day is commonly celebrated in both the Theravadin and Tibetan traditions. I have to admit that I haven't heard of it being celebrated in the Zen tradition (so it's not, at least in the West but I also always missed the Bathing of Lord Buddha and Queen Maya would have to have been mentioned then) and I don't know about the other Mahayana traditions.

Descent from Heaven Day is the day Lord Buddha returned from the Heaven of the 33 where his mother had been reborn. He went there to teach her and the devas living there.

"Even if you practice only for an hour a day with faith and inspiration, good qualities will steadily increase. Regular practice makes it easy to transform your mind. From seeing only relative truth, you will eventually reach a profound certainty in the meaning of absolute truth."Kyabje Dilgo Khyentse Rinpoche.

Other then giving Birth to Buddha she didn't really do anything of great Importance compared to Buddha's enlightenment and turning of the wheel of Dharma. Only Buddha experienced his mothers kindness where as all Buddhist practitioners experience Buddha's kindness through his setting the wheel in motion.

Abandoning Dharma is, in the final analysis, disparaging the Hinayana because of the Mahayana; favoring the Hinayana on account of the Mahayana; playing off sutra against tantra; playing off the four classes of the tantras against each other; favoring one of the Tibetan schools—the Sakya, Gelug, Kagyu, or Nyingma—and disparaging the rest; and so on. In other words, we abandon Dharma any time we favor our own tenets and disparage the rest.